1. Field of the Invention
The present invention relates in general to a control apparatus for controlling an anti-lock or anti-skid brake system for a vehicle, and more particularly to a technique for improving the accuracy of control of the control apparatus.
2. Discussion of the Prior Art
The anti-lock brake control system generally includes (a) a plurality of wheel speed sensors for detecting actual speeds of respective wheels of a motor vehicle, (b) an electrically controlled brake pressure regulating device having at least one solenoid-operated valve for regulating the braking pressure applied to each of the wheels, and (c) a controller for controlling the pressure regulating device so as to prevent locking or skidding of the wheel on a road surface, on the basis of a relationship between the actual speed of the wheel detected by the corresponding wheel speed sensor, and the running speed of the vehicle.
An example of the anti-lock brake control apparatus of the type indicated above is known, for example, as a multiple-mode type as disclosed in laid-open Publication No. 2-129960 of unexamined Japanese Utility Model Application. In the multiple-mode type control apparatus disclosed in this publication, the pressure regulating device having the solenoid-operated pressure regulating valve or valves is operable in different pressure control modes such as a rapid pressure-increase mode, a slow pressure-increase mode, a pressure-hold mode, a rapid pressure-decrease mode and a slow pressure-decrease mode, and the controller estimates the vehicle running speed on the basis of the detected actual speeds of the wheels, at a predetermined cycle time, so that the controller selects one of the pressure control modes that is most suitable for restoring the actual speed of each wheel to a target or desired speed, depending upon the relationship between the estimated vehicle speed and the detected speed of the wheel in question. The controller activates the pressure regulating device in the selected pressure control mode for regulating the braking pressure to be applied to the wheel.
In the known anti-lock brake control apparatus constructed as described above, the running speed of the vehicle is estimated based on the detected speeds of the plurality of wheels, on the assumption that the detected speed of at least one of the wheels is equal to the actual running speed of the vehicle. However, this arrangement does not assure a sufficiently high degree of accuracy in estimating the vehicle speed, since the above assumption is not always true.
The known anti-lock brake control apparatus is also adapted to select one of the plurality of different pressure control modes that permits to establish the most ideal braking pressure gradient, with high precision, for restoring the actual speed of each wheel to its desired or target speed. Since the number of the pressure control modes available is limited, the selected mode is not always most ideal for controlling the gradient along which the braking pressure is regulated. In this sense, too, the known control apparatus is not sufficiently satisfactory in the precision of regulation of the braking pressure for each wheel.
Thus, the known anti-lock brake control apparatus more or less has difficulty in assuring sufficiently high degrees of accuracy in the estimation of the vehicle running speed and in the selection of the pressure control mode. In other words, the known arrangement suffers from an undesirable deviation in the estimated vehicle speed with respect the actual vehicle speed (e.g., ground-speed of the vehicle relative to the road surface), as indicated in the graph of FIG. 24, whereby the braking pressure controlled on the basis of the estimated vehicle speed tends to overshoot, resulting in insufficient precision of control of the braking pressure. Accordingly, the known arrangement also suffers from relatively frequent operations of the solenoid-operated valves, which lead to high levels of operating noise and vibration due to the frequent operations of the valves and the related components.